(a) Technical Field of the Invention
The present invention generally relates to backlight modules for liquid crystal display devices, and more particularly to a backlight module using LEDs to emit light into a light mixing plate from the sides.
(b) Description of the Prior Art
Liquid crystal display (LCD) devices were applied mainly in notebook computers and LCD monitors, whose requirement for the backlight modules is focused on light weight, small form factor, and thin thickness. On the other hand, for large-size LCD devices such as LCD TVs, their backlight modules are further required to provide high luminance (at least 450 cd/m2), a wide viewing angle, crispy contrast, and long operational life. As such, direct-lit backlight modules have become the mainstream technology for large-size LCD devices, as they are able to provide planar light having the required high luminance, high uniformity, and a wide viewing angle.
A conventional direct-lit backlight module 1, as, shown in FIG. 1, contains at least a casing 11, multiple lamps 12, a diffusion plate 13, and a number of optical sheets 14. For ease of reference throughout this specification, the terms ‘front’ and ‘back’ are used to refer to locations along the path of light that are closer to and farther away from the light source (i.e., lamps) respectively.
The casing 11 is a tray having an opening 111. The opening 111 gradually shrink to the inside of the casing 11. The inner surface of the casing 11 is reflective or is coated with a reflection film 112.
The lamps 12 are usually cold cathode fluorescent lamp (CCFL) tubes arranged in parallel inside the casing 11.
The diffusion plate 13 is a flat object embedded with diffusion agents or diffusion beads, so as to scatter and uniform the light emitted from the lamps 12. The diffusion plate 13 is positioned along the path of light from the lamps 12 and covers the opening 111 of the casing 11.
The optical sheets 14 include one or more diffusion sheets 141 and prism sheets 142, whose functions are for further scattering the light to achieve better uniformity and for focusing the light into a proper viewing angle to achieve enhanced brightness.
To assemble the conventional direct-lit backlight module 1, the lamps 12 are first positioned inside the casing 11. The diffusion plate 13 is then positioned at the opening 111 of the casing 11. At last, the diffusion sheets 141 and the prism sheets 142 are stacked behind the diffusion plate 13 along the path of light. The assembled direct-lit backlight module 1 is illustrated in FIG. 2.
When the direct-lit backlight module 1 is in operation, the lamps 12 radiate light as proper electrical voltage is applied. A portion of the light directly propagates toward the diffusion plate 13. The other portion of the light is reflected by the inner surface of the casing 11 and redirected to the diffusion plate 13. As the light passing through the diffusion plate 13, it is scattered and thereby uniformed by the embedded diffusion agents or diffusion beads. As the light continues to propagate through the diffusion sheets 141 and the prism sheets 142, it is further scattered for uniformity enhancement and then focused into collimated light beams within a proper viewing angle with enhanced brightness.
The foregoing direct-lit backlight module 1 has been proven to be an effective solution and has been put into use in real products. However, the mercury contained in the CCFL tubes presents an environmental hazard during fabrication and recycling as well. Many countries have already been making laws to outlaw the use of mercury in products and goods. As such, CCFL-based direct-lit backlight modules are gradually replaced by direct-lit backlight modules using light emitting diodes (LEDs) as the light source.
As illustrated in FIG. 3, a LED-based direct-lit backlight module 2 mainly contains a casing 21, a number of LEDs 22 as the light source, a light mixing plate 23, a diffusion plate 24, and one or more optical sheets 25.
The casing 21 is a tray having an opening 211. The opening 211 gradually shrink to the inside of the casing 21. The inner surface of the casing 21 is reflective or is coated with a reflection film 212.
The LEDs 22 contain red-light (R) LEDs 221, green-light (G) LEDs 222, and blue-light (13) LEDs 223, and these LEDs are sequentially arranged in lines inside the casing 21.
The light mixing plate 23 is positioned along the path of lights from the LEDs 22 and covers the opening 211 of the casing 21. The light mixing plate 23 is made of a material having high transparency (such as PMMA). The light mixing plate 23 has a light emission plane 231 and a light incidence plane 232. Upon one of the light emission and incidence planes 231 and 232, a number of light shielding dots 233 are coated at locations corresponding to the LEDs 22. The light shielding dots 233 are made of a coating material that can significantly shield the light of the LEDs 22 from penetration.
The diffusion plate 24 and the optical sheets 25 are positioned at the opening 211 of the casing 21 behind the light mixing plate 23. The optical sheets 25 contain one ore more diffusion sheet 251 and prism sheets 252. The number of diffusion and prism sheets 251 and 252, and their relative positions, could be adjusted based on the application requirement.
The light from the LEDs 22 is blocked by the light shielding dots 233 immediately in the front, and therefore a large portion of the light propagates along the inside of the light mixing plate 23. As such, the colored lights from the red-light LEDs 221, green-light LEDs 222, and blue-light LEDs 223 are mixed inside the light mixing plate 23 to produce white light. The lights are then further scattered and uniformed by the diffusion plate 24 and the diffusion sheets 251. The lights are also focused by the prism sheets 252 for brightness enhancement, and then are projected to the back of the LCD panel A.
As shown in FIG. 4, as the LEDs 22 are positioned outside of the light incidence plane 232, light from the LEDs 22 are incident into the light incidence plane 232 at an angle. Even though a portion of the light indeed propagates along the light mixing plate 23 and is thereby mixed, still a large portion of the non-mixed, red, green, and blue lights is directly refracted out of the light emission plane 231 if their incident angles to the light emission plane 231 are smaller than a threshold angle. This incomplete mixing phenomenon could be resolved by lengthening the distance between the light mixing plate 23 and the diffusion plate 24 so that these non-mixed lights get a second chance to mix with each other as they propagate toward the diffusion plate 24. This inevitably makes the backlight module 2 quite thick, which is not conforming to the market's requirement for slim LCDs.
To overcome the foregoing problem of LED-based direct-lit backlight modules, a technique illustrated in FIG. 5 has been disclosed. As shown, the backlight module 3 similarly contains a casing 31, a number of LEDs 32, a light mixing plate 33, a diffusion plate 34, and one or more optical sheets 35. The difference lies in that the LEDs 32 are side-emitting LEDs, as shown in FIG. 6. Each of the LEDs 32 has a reflection lens 321 in the shape of an inverted cone configured in the front, which reflects the light from the LED 32 and the light's incident angle into the light incidence plane 332 of the light mixing plate 33 is thereby increased. This technique is effective but, however, only to a limited extent. Still a large portion of the non-mixed lights from the LEDs 32 is refracted out of the light emission plane 331 and a certain distance between the light mixing plate 33 and the diffusion plate 34 still has to be maintained.